XAUEUR is the price of one troy ounce of gold in euros, derived from XAUUSD divided by EURUSD. For a euro-denominated account it is the honest measure of gold performance, because XAUUSD can rise purely on dollar weakness.
The gold leg follows US real yields and risk appetite while the euro leg follows ECB policy, eurozone inflation, German data and European energy and political risk. Fed and ECB divergence is what produces the strongest XAUEUR trends.
Liquidity is best from the European morning (07:00 UTC) through the New York overlap to 17:00 UTC, and thin in the Asian session. ECB decision days widen the spread severely.